A normal eyeglass frame comprises a central bridge or nose piece, a pair of upper lens-holding arms, and a pair of lower lens-holding arms that together with the respective upper arms embrace and hold respective lenses. While the inner ends of the arms are secured to each other or to the nose piece, and in fact can be joined unitarily together so that in effect each upper and the respective lower arms are simply sections of the same U-section wire, the outer ends are normally joined together at a front tube. To form this structure the outer end of each lower arm is normally formed with a threaded hole and the outer end of the upper arm with an unthreaded hole, so that a screw can be inserted through these two ends to secure them together and lock the respective lens solidly between the arms, bringing the outer arm ends into solid engagement with one another.
Somewhat behind each front tube formed by or secured to the outer arms ends there are two extensions that form a second pair of eyes or rear tube to which the front end of the respective temple is mounted. To this end the temple end is normally inserted between the two rear eyes, one of which is threaded, and another screw is inserted through them, forming a pivot for the temple. Thus at each side of the frame there are two tiny screws, one of which locks the two arms around the respective lens and the other of which acts at the pivot for the respective temple piece.
To assemble such a frame both screws must be withdrawn and the lens-holding arms spread since the frames are often marketed to the optician with dummy lenses that give the customers realistic glasses to try on and that hold the shape of the frames. The customer's prescription lenses are fitted between the respective arms and the front screws are driven home while the technician carefully holds the arms together to retain the lens in place until the screw is seated. Then the temples are set in place and the rear screw is installed.
This is obviously a cumbersome process. It is necessary to keep track of the tiny screws and various parts of the frame during assembly. Special tools are needed so that it is normally not considered practical for the consumer to change lenses himself or herself. Another disadvantage is that the frame and lenses must be a perfect fit; if the lenses are too small they will not be gripped tightly and will fall out even when the respective screw is tightened all the way down and if they are too big the outer arm ends will not meet so they cannot be screwed together.
In addition such a frame is relatively expensive to manufacture. It has numerous parts that must be accurately formed and machined, and tiny screwthreads must even be cut into some of them. What is more, in use it is fairly common for the screws to work loose, allowing the lenses to drop out and the screws to get lost, requiring professional repair.